What a load of b0ll0x - what it probably does prove is that 2nd & 3rd generation immigrant families, who probably speak their 'mother tongue' at home so as to include the grand parents in their conversations, are having more children than the 'natives' in their particular areas.
And it almost certainly also includes those schools in Wales where the pupils would claim Welsh as their first language.
Yet more right wing bluster and posturing. There is NOTHING in that article to show that these children are, in fact, immigrants, other than the mention of 10,000 eastern Europeans ( but not of what proportion might be of school age ). When I was at school, we had a lad whose mother was German, and they spoke her native tongue at home, but he was most definitely English.